High-strength polyester fiber is being variously used for industrial applications such as rubber reinforcing tire cord, seat belts, conveyor belts, V-belts and hoses. In particular, for fiber reinforcement of the tire, wherein latex-treated and heat-treated tire cords are applied, superior dimensional stability and tenacity are desired.
Conventionally, in order to increase the tenacity of the polyester fiber used for industrial purposes, the temperature of a high viscosity chip having intrinsic viscosity of 1.0 or more was sufficiently elevated to 300° C., so that the chip was melted and solidified. An undrawn yarn, which was produced by winding at the winding speed of 200 m/min or more in a Godet roller, was drawn by going through one or two stages and was allowed to relax.
However, in the method as described above, it was difficult to produce a yarn due to unwindability defects such as drawability decrease or filament fusing caused by increasing crystallinity.
Further, conventionally, in order to increase the tenacity of a polyester fiber used for industrial purposes, a method of manufacturing a yarn with a high draw ratio was used, but this method caused the problem that dimensional stability of a cord at a high temperature decreased and thus durability of the tire became lower, because of structural defects such as excess orientation and uneven length of the molecular chains of tie chain.